Field of the Disclosure
The present disclosure relates generally to wireless communication systems and, more particularly, to synchronization of base stations in wireless communication systems.
Description of the Related Art
Wireless communication systems include base stations for providing wireless coverage within corresponding cells. The base stations may also be referred to as eNodeBs, base station routers, and the like. As used herein, the term base station may also refer to small cells such as access points, home base station routers, metrocells, microcells, femtocells, picocells, and the like. User equipment in the wireless communication system may be located using signals transmitted by the base stations. For example, the Observed Time Difference Of Arrival (OTDOA) technique uses the difference in arrival times of position reference signals (PRSs) transmitted by multiple base stations to a user equipment to estimate the location of the user equipment by multilateration. Base stations can transmit the PRS in one of a standardized set of configurations that are defined by the cell specific subframe configuration period, the cell specific subframe offset for PRS transmission, and the number of consecutive PRS downlink subframes. Base stations may also be configured with PRS muting patterns. When one base station transmits PRS, the corresponding PRS resources for the neighboring base station are muted to reduce the interference to the PRS, so that the user equipment can listen to the transmitted PRS with reduced interference. The configurations of the PRS may be different for different base stations and particular configurations are conveyed to user equipment when OTDOA measurements are requested. For another example, in the Uplink Time Difference Of Arrival (UTDOA) technique user equipment transmit sounding reference signals (SRSs) to the base stations and the location of the user equipment is calculated using differences in arrival times of the SRSs received by the base stations.
The accuracy of OTDOA or UTDOA position determinations depends on the degree of synchronization between the timing references used by the different base stations. Consequently, base stations that implement OTDOA or UTDOA must satisfy stringent timing constraints on phase synchronization, delay measurements, and delay compensation. For example, U.S. Federal Communication Commission (FCC) requirements for the E911 service specify that an emergency caller's location be determined within an accuracy of at least 50 meter (m) for 67% of calls and an accuracy of at least 150 m for 90% of calls. To achieve the FCC-mandated accuracy, transmission times of downlink radio frames from antenna tips at different base stations should normally be synchronized within 100 nanosecond (ns) accuracy.